Electrochemical Reduction of Nitric Oxide with 1.7% Solar‐to‐Ammonia Efficiency Over Nanostructured Core‐Shell Catalyst at Low Overpotentials

Abstract Transition metals have been recognized as excellent and efficient catalysts for the electrochemical nitric oxide reduction reaction (NORR) to value‐added chemicals. In this work, a class of core–shell electrocatalysts that utilize nickel nanoparticles in the core and nitrogen‐doped porous carbon architecture in the shell (Ni@NC) for the efficient electroreduction of NO to ammonia (NH3) is reported. In Ni@NC, the NC prevents the dissolution of Ni nanoparticles and ensures the long‐term stability of the catalyst. The Ni nanoparticles involve in the catalytic reduction of NO to NH3 during electrolysis. As a result, the Ni@NC achieves a faradaic efficiency (FE) of 72.3% at 0.16 V RHE. The full‐cell electrolyzer is constructed by coupling Ni@NC as cathode for NORR and RuO2 as an anode for oxygen evolution reaction (OER), which delivers a stable performance over 20 cycles at 1.5 V. While integrating this setup with a PV‐electrolyzer cell, and it demonstrates an appreciable FE of >50%. Thus, the results exemplify that the core–shell catalyst based electrolyzer is a promising approach for the stable NO to NH3 electroconversion.

Thermogravimetric Analysis (TGA) was carried out by TGA/Auto Q500 under an inert atmosphere. The CHNS analysis was carried out using an elemental analyzer (Vario MICRI cube) setup to quantify carbon and nitrogen in the catalysts. The porous nature of all the catalysts as characterized by BET, Micromeritics ASAP 2020. The Bruner-Emmett-Teller method was used to calculate the surface area and pore size distribution. All solar-driven electrolysis experiments were carried out using a conventional solar panel (GaAs thin-film solar cell) powered by a solar simulator (K3300ELX/ Cytec Korea) to illuminate simulated sunlight (1 Sun, AM 1.5 G). The current consumed during electrolysis was monitored by a 2636A-digital source meter (Keithley).
Electrochemical characterizations: The entire NORR study was conducted using a multichannel potentiostat (Biologic, VSP) and air-tight H-type electrochemical cell separated by the Nafion-212 membrane. The Nafion membrane was pretreated with H 2 O 2 / DIW mixture (1:5) at 100 °C for 1 h to remove the organic impurities, followed by subsequent rinsing and boiling in DIW. Then, the membrane was boiled in H 2 SO 4 (0.5 м) for 1 h for protonation and finally washed with DIW. The catalyst-loaded GDE, graphite rod, and Ag/AgCl (saturated KCl electrolyte) were used as working, counter, and reference electrodes. Before starting each electrolysis, the ohmic resistance between the working electrode and the reference electrode was estimated using electrochemical impedance spectroscopy (EIS) between 200 kHz and 1 Hz with an amplitude of 10 mV. The resistance value was then determined by the intersection of the curve with the Real (Ω) axis in the Nyquist plot. iR correction was performed after electrolysis for all measurements. Every measurement was performed on a freshly prepared electrode. Current densities were calculated based on the catalyst-covered geometric area of the working electrode. The polarization curves were obtained at a scan rate of 5 mV s -1 and all the potential values were converted to RHE scale using the Nernst relation (E RHE = E WE + E°A g/AgCl + 0.059 pH, E°A g/AgCl = 0.197 V). All the experiments were carried out at room temperature (~25°C). To carry out electrolysis, the two chambers of the H-cell were filled with HCl (0.1 м). To ensure the complete elimination of dissolved oxygen, the cathodic compartment was purged with high purity Ar gas for at least 1 h. For the entire study, the high concentrate NO (100 %) was used as a source gas and purged (at rate of 1 sccm) with a suitable sparger in the cathode compartment. The headspace was covered by Ar flow to prevent NO 2 formation and O 2 dissolution. To capture the possible ammonia gas product, the tail gas of the catholyte chamber was trapped in an acidic HCl solution. After the electrolysis, the excess dissolved NO gas was removed by purging the Ar gas. For the full-cell experiments, catalyst-coated GDE was used as NORR electrocatalyst, and RuO 2 (20 wt%) was used as the OER electrocatalyst with areal loading of 1 mg cm -2 . NORR-OER electrolysis was performed in the same two-compartment cell controlled by the potentiostat in the twoelectrode configuration or powered by a conventional solar panel illuminated with simulated sunlight.
Electrochemical active surface area: Electrochemical active surface area (ECSA) of all catalysts was determined from the double-layer capacitance (C dl ) in a non-Faradic region using a typical cyclic voltammetry (CV) method. The double-layer current (i) is equal to the product of the scan rate (v) and C dl (i = vC dl ), which is expected to be linearly proportional to the ECSA of the electrode. The C dl was determined as half of the slope by plotting the capacitive currents (ΔJ, J anodic -J cathodic /2) versus v. The difference between the anodic and cathodic current was obtained at the potential of 0.66 V RHE . Finally, the ECSA was estimated by dividing the C dl by the specific capacitance (ECSA=C dl /C s , here the C s value of 0.035 mF cm -2 for HCl (0.1 м) was used, based on the reported average C s of Ni-based catalyst in acidic solution).
Electrode fabrication: The spray coating technique was applied to spray catalyst ink on GDE (1 x 1 cm 2 ) until 1 mg cm -2 loading was achieved. The catalyst ink was prepared by dispersing 5 mg of catalyst in IPA (500 μL), DIW (100 μL), and 5 wt% Nafion solution (50 μL). The resultant solution is sonicated for 45 min to form a homogeneous catalyst ink.
Product quantification: Possible gaseous products such as H 2 , N 2 , N 2 O were not considered during the entire NORR electrolysis. However, tail gas from the cell was introduced into the acid trap to monitor gaseous NH 3 . NH 3 was quantified using the indophenol blue method and 1 H NMR. NH 2 OH and N 2 H 4 were estimated using colorimetric methods. The NH 3 was determined spectroscopically using indophenol blue method. [2] After 1 h of electrolysis, 2 mL of analyte was mixed with Solution A (2 mL), Solution B (1 mL), and Solution C (200 μL) (Solution A: NaOH (1 м) containing salicylic acid (5 wt.%), trisodium citrate dihydrate (5 wt.%); solution B: sodium hypochlorite (0.05 м); Solution C: 1 wt.% Sodium nitroprusside).
After 1 h incubation at dark, absorption at 655 nm was taken using UV-vis spectrophotometer to calculate the ammonia yield. Similarly, ammonium chloride of known concentration was dissolved in HCl (0.1 м) to obtain a standard calibration plot. To quantify ammonia using 1 H NMR, analyte (400 μL) was blended with H 2 SO 4 (50 μL, 4 м) and DMSO-d 6 (50 μL). Maleic acid of known concentration was added as an internal standard to obtain quantifiable data. At the same time, the ammonia was calculated by integrating the triplet with respect to the standard maleic acid peak (6.25 δ). Similarly, 1 H NMR spectra of electrolytecontaining known concentrations of ammonium chloride were derived to draw a linear plot. It is important to note that the acquisition with ordinarily applied pulse sequence (zg30) did not deliver any characteristic triplet peaks of ammonia except if the number of scans was higher (>512). This is because of dominating proton signals from water since it is an aqueous solvent.
To eliminate this issue, sculpting pulse sequence (zgesgp) was employed along with the solvent suppression having a relaxation delay (d1) of 3 s and 256 scans during acquisition.
The amount of hydrazine in the electrolyte was quantified by the Watt and Chrisp method. [3] At first, the color reagent was prepared by mixing para-(dimethylamino) benzaldehyde (5.99 g) in concentrated HCl (30 mL) and ethanol (300 mL). Analyte (2 mL) was diluted with DIW and blended with KOH (1 mL, 1 м), followed by adding color reagent (5 mL). After incubated in the dark for about 10 min, the absorbance at a wavelength of 455 nm was taken to calculate the amount of hydrazine formed. In the same way, the absorption-concentration curve was obtained by dissolving a known concentration of hydrazine hydrate in HCl (0.1 м) electrolyte. NH 2 OH was quantified by a colorimetric method. [4] In brief, analyte (1 mL), phosphate buffer solution (1 mL, 0.05 м), DIW (0.8 mL), trichloroacetic acid (0.2 mL), 8-quinolinol (1 mL) were taken and swirled gently, followed by adding Na 2 CO 3 (1 mL, 1 м).
The resultant solution was shaken vigorously with a stopper; finally, the tube was kept in a boiling water bath for color development. Then, the solution blend was cooled at room temperature for 15 min, and the absorbance was taken at a wavelength of 705 nm to evaluate concentration. Following the same procedure, a calibration curve was established by testing a series of NH 2 OH.HCl solutions in the concentration range of 6-40 μм.

Equations used for the calculation:
The average yield rate (Y p ) of the product was calculated as follows: Where Y p is the products formation rate (µmol cm -2 h -1 ), V is the total volume of electrolyte (mL), A is the electrode area (cm 2 ), t is time (h) for NORR, and M w is the molar mass of the product (g mol -1 ).
The Faradaic Efficiency (FE) can be calculated using the following formula: Where n is the number of electrons transferred, F is the faraday constant (96485 C mol -1 ), Y p is the products formation rate (µmol cm -2 h -1 ), t is time (h) for NORR, A is the electrode area (cm 2 ), and Q is the quantity of charge consumed during the reaction (C).
The Energy Efficiency (EE) can be calculated using the following formula: Where E o is the theoretical thermodynamic cell voltage (V), and E c is the applied cell voltage The STF can be calculated using the following formula: Where E o is the theoretical thermodynamic cell voltage (V), J solar is the solar current density (mA cm -2 ), and P solar is the solar energy input (mW cm -2 ).
The turnover frequency (TOF) was calculated as follows: Where i is the current density (mA cm -2 ), M w is the molecular weight of nickel (g mol -1 ), is the catalyst loading (mg cm -2 ), and F is the faraday constant (96485 A s mol -1 ).

Note S1. Determination TOF at an overpotential of 550 mV for all Ni@NC catalysts
The amount of Ni loading in the catalyst is measured using ICP-OES (Table S4). The following equation is used to calculate the TOF value, The NORR current density at the overpotential (ŋ) of 550 mV was taken for all the catalysts,